1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as copying machines, photoprinters and facsimile apparatuses employing an electrophotographic system.
2. Description of the Related Art
In conventional image forming apparatuses using an electrophotographic system, in the case of utilizing a transfer device of a corona discharge system, a transfer current is applied with appropriate timing to a recording material which has been fed to a transfer position between a photoreceptor and the transfer device, whereby a toner image on the photoreceptor is transferred onto the recording material.
However, feeding variation of recording material or timing variation of transfer signal causes application of the transfer current before arrival of a recording material at the transfer position between the photoreceptor and the transfer device, with the result that the transfer current with a polarity opposite to the electrostatic charge in forming a latent image is directly applied to the photoreceptor, which causes an image defect of xe2x80x9ctransfer memoryxe2x80x9d. This is because during printing on a sheet of the recording material, the photoreceptor rotates more than one time for printing one sheet of recording material.
Japanese Unexamined Patent Publication JP-A 7-239607 (1995) has proposed an image forming apparatus wherein damage from a transfer drum in a transfer process is reduced by extending the diameter of an image carrying member to form a quality image. The image forming apparatus is hereinafter referred to as xe2x80x9cprior art 1xe2x80x9d.
Japanese Unexamined Patent Publication JP-A 10-78712 (1998) has proposed an image forming apparatus wherein by applying to a transfer roller which is always in contact with a photoreceptor drum, a high level voltage and a low level voltage separately, responsive to pass and non-pass of the recording material at the application of the voltage, a transfer memory inhibiting effect is provided (hereinafter referred to as xe2x80x9cprior art 2xe2x80x9d).
With respect to the prior art 1, however, by extending the diameter of the photoreceptor, the overall apparatus will be bigger, whereby the transfer memory occurs due to an unavoidable feeding variation of recording material or the like. Further, extending the diameter of the photoreceptor goes against the downsizing of the apparatus and cost-lowering trend. Furthermore, a non-contact transfer device having a corona discharge system, does not have a feeding accuracy like a contact-type transfer roller, namely, the feeding accuracy for accurately feeding the recording material to the transfer point. Thus, the effect of unavoidable feeding variation and transfer the signal timing variation is very severe.
With the prior art 2, the voltage application timing in the high-level state is made earlier by a rise time of a transfer bias power source so that the front edge of the recording material reaches the transfer point at a time when a transfer output is sufficiently raised. In this art, however, a feeding variation of the recording material is not considered at all. Thus, even if a few variations are caused, the transfer memory occurs.
Hence, an object of the present invention is to provide an image forming apparatus which prevents the occurrence of the transfer memory by ensuring a sufficient margin against variation without limiting the diameter of the photoreceptor.
The present invention provides an image forming apparatus including an image carrier; and an electrostatic charging device for causing the image carrier to be electrostatically charged. An exposure device is further included for forming an electrostatic latent image by exposing the electrostatically charged image carrier to a light. A developing device then develops the formed electrostatic latent image with toner. Further, a transfer device is included for transferring a toner image formed by development, to a recording material which is in contact with a peripheral surface of the image carrier, by generating the corona discharge directed to the recording material. Finally, a current controller controls a transfer current for corona discharge applied to the transfer device so as to be applied stepwise to the transfer device at a time of start-up of the transfer.
According to the invention, the stepwise current application to the transfer device makes it possible to prevent the transfer memory from being formed on the photoreceptor as the image carrier, and to transfer an image properly.
In addition, in the invention, it is preferable that the current controller controls the transfer current so as to be applied stepwise, by adjusting on and off time durations of a current applying control signal composed of on and off signals. As such, a control system of the apparatus can be simplified.
According to the invention, a control system of the apparatus can be simplified.
Furthermore, in the invention, it is preferable that the current controller controls the transfer current so as to be applied stepwise by using a switching circuit system to adjust an amount of time of application of the transfer current to the transfer device. As such, finer and complicated control is possible.
Furthermore, in the invention, it is preferable that the current controller performs the stepwise application of the transfer current to the transfer device in a process for raising a current value to a predetermined level at which a constant-current control is possible and a number of transfer current steps is one or two. By defining the number of transfer current steps as one or two, no transfer memory occurs and a favorable edge transfer performance can be maintained.
Furthermore, in the invention, it is preferable that the current controller performs the stepwise application of the transfer current to the transfer device in a process for raising a current value to a predetermined level at which constant-current control is possible, and a duration of the process is 100 to 120 ms. As such, no transfer memory occurs, and the favorable edge transfer performance can be maintained.
Furthermore, in the invention, it is preferable that the current controller performs the stepwise application of the transfer device in the process for raising a current value to a predetermined level at which constant-current control is possible, and the stepwise transfer current application is performed when a current value is within a range of 40% to 50% of the predetermined level. As such, no transfer memory occurs, and a favorable edge transfer performance can be maintained.
Furthermore, in the invention, it is preferable that the image forming apparatus further includes a humidity sensor for sensing a humidity of an operational environment, wherein the current controller varies stepwise applying conditions of the transfer current based on a result sensed by the humidity sensor. As such, stabilized transfer performance can be maintained without being influenced by environmental conditions, particularly humidity.
Furthermore, in the invention, it is preferable that the image forming apparatus further includes a thickness sensor for sensing a thickness of the recording material, wherein the current controller varies stepwise applying conditions of the transfer current based on a result sensed by the thickness sensor. As such, stabilized transfer performance can be maintained without being subjected to the thickness of the recording material.
Furthermore, in the invention, it is preferably that the transfer device is provided with a corona discharge electrode composed of a wire electrode. As such, a transfer device having a simplified structure and good volume production can be provided.
Furthermore, in the invention, it is preferable that the transfer device is provided with a corona discharge electrode composed of a serrated electrode. As such, the amount of generated ozone can be reduced. Thus, an environment-friendly transfer device can be provided in comparison with the case of using the wire electrode.